An investigation into the characterization of the thermodynamic equilibrium between the alpha-Al2O3 and Na-beta-alumina phases as a function of sodium activity is reported here for the first time using a novel approach. A sodium reference electrode comprising vitreous NaAlSi3O8 along with Al2O3 and SO2 in conjunction with an yttria stabilized zirconia (YSZ) disk was used to vary the oxygen potential and in turn, the sodium potential, by application of a potential difference across YSZ. This system was found to be promising for use as a potential sodium reference electrode. The results from the present investigation showed there exists a very narrow or no window of sodium activity over which the sodium oxide activity in a alpha-Al2O3-/beta-Al2O3 system remains invariant unlike a wide range found for the beta/beta"-alumina system. (C) 2003 Elsevier Ltd. All rights reserved.